Surface inspection system

ABSTRACT

A surface inspection system utilizes a laser light source, and a vacuum chuck to support the material whose surface is to be inspected. The vacuum chuck is rotatable. The laser beam may be translated across the work surface while the chuck is rotated, all the while maintaining mutual perpendicularity of the vacuum chuck work surface and the laser beam. An airtrack transports the material whose surface is to be inspected. Sensors are utilized to detect the position of such material along the track and to guide the system in its operation.

This invention relates to a surface measurement system using collimatedlight which impinges upon the surface to be measured and a detectorwhich detects the light scattered by surface irregularities.

In another respect, the invention relates to a surface measuring systemusing a laser light source for measurements of surface irregularities onhighly polished planar surfaces.

In still another respect, the invention relates to a surface inspectionsystem for use with semiconductor wafers providing control means fortransporting said wafers to the location at which the surfacemeasurement is to be made and away from that site upon completion ofsaid measurement.

In yet another respect, the invention relates to a scattered light,surface inspection system, utilizing a laser light source and ascattered light detector, which employs no reflective surfaces in thelight path between the laser output, the surface to be measured, and thescattered light detector.

In particular, the invention relates to a surface inspection systemwhich rotates a highly polished planar surface and simultaneouslytranslates that surface relative to an impinging beam of collimatedlight such that the light beam scans the surface being measured so thatthe track of said beam on said surface describes an archimedes spiral.

The rapid advancement of growth of technology in the semiconductor fieldhas affected the lives of everyone. Semiconductors are now found notonly in equipment considered to be "electronic" in nature such asreceivers and computer equipment, but they are finding themselves intoeveryday household items. Children's toys, the homeowner's tools, smallappliances for use in kitchens, and the like may all be found today tocontain some form of semiconductor either to control speed, provideguidance, or to program a whole sequence of operations. The art ofproviding high-quality process semiconductor material has grown apacewith the technological innovations which lead to ever greater and variedusage of semiconductor devices.

Processed semiconductor materials of high quality, at reasonable costs,are required to support the burgeoning semiconductor industry.Manufacturing processes require tight quality control procedures whichare highly efficient if the flow of processed semiconductor materials atreasonable prices is to be realized. The present invention is responsiveto meeting the need of one such highly precise and efficient qualitycontrol procedure. The surface quality of semiconductor wafers beingprocessed for use in semiconductor industries must be constantlymonitored for defects introduced during the handling and processing ofthe wafers. The invention herein disclosed finds broad applicationduring such semiconductor wafer processing in the areas ofincoming/outgoing quality control, in-process quality assurance,polishing and cleaning process control, pre-oxidation andpost-oxidation/photolithography/diffusion control, and pre/postepitaxial process control.

Semiconductor materials are provided to semiconductor devicemanufacturers in the form of wafers. Wafer sizes in the range of two tofive inches are typical. A conventional thickness would be about 0.025inches.

Surface quality of the wafers is a critical criterion which must becontrolled if high-yield semiconductor devices are to be produced. It isnot unusual to find semiconductor processors utilizing equipment capableof detecting surface defects in the planar, polished surface of asemiconductor wafer, which defects have physical dimensions on the orderof one micron (the thousandth part of one millimeter). It is a challengeto utilize such precision equipment under the demands generated by ahigh production, controlled quality environment.

The concept of determining surface quality using scattered lighttechniques is old in the art. The principle relied upon is the familiarone that the angle at which light is incident on a smooth, planarsurface is equal to the angle at which the light reflects from thatsurface. The proposition is often stated more succinctly as, "the angleof incidence equals the angle of reflection." A collimated light sourceis assumed. Since the angle of incidence and the angle of reflection areequal, it is readily seen that as the light approaches the surface froman incidence angle of 90°, the reflected light will leave the surface ofthe reflector and travel back along the path of the incident light.However, should a superficial anomaly appear on the surface at the pointat which the light is incident, there will tend to be a scattering ofthe light as it strikes the anomaly and light will be reflected atvarious angles different from that at which the light is incident uponthat portion of the surface having no such anomalies.

To understand the basic mechanism employed to detect surface defects,one should envision a light beam from a collimated light source. Thebeam passes through an aperture and is directed toward the surface to beinspected so as to be incident on that surface at an angle of 90°. Ifthe surface is truly planar, light reflected from it will travel backalong the path of incidence, passing back through the aperture openingas it does so. However, should the surface be defective in some way soas to cause scattering of the light as it impinges on the surface, thereflected light will be scattered and will not pass back through theaperture opening. Rather, some of the reflected light will strike thesurface surrounding and supporting the aperture opening. The greater thesurface anomaly, the greater will be the scattering of light uponreflection from that surface and the greater will be the amount of lightwhich misses the aperture opening and strikes the surface surroundingand supporting that aperture. If that surface is comprised of a lightsensor, then the magnitude of the output from that light sensor willquantitatively indicate the magnitude of the surface anomaly.

A laser is the most frequently used source of collimated light. Thelight is directed from the laser output via a series of prisms orreflective surfaces so as to impinge on the surface to be measured atthe required 90° angle. The surface to be measured is then passedbeneath the laser beam and the degree of light scattering determined. Amajor problem in devising such a system for measuring surface qualityconcerns the maintenance of the light beam and the surface to bemeasured at the critical 90° angle of incidence. The adjustment of thelaser beam, the establishment of proper working angles for prisms andreflectors, and the control of the relative movement between laser beamand measured surface are all of major importance before the firstmeasurement can be made. Maintaining these relationships over any periodof time and use of the equipment present an almost insurmountableproblem. In the processing of semiconductor materials surface quality isof prime importance. For economies of operation the semiconductormaterials must pass smoothly and efficiently through the surfaceinspection devices so that cost effective production may be maintained.

It is an object of the present invention to provide a surface inspectionsystem for the quantitative measurement of the defect level present on ahighly reflective surface.

It is a further object of the invention that it should incorporate ahighly sensitive solid-state detector to collect laser energy that hasbeen scattered by defects such as surface haze, particles, scratches,fingerprints, moisture, hillocks, spikes, etc.

An additional object of the invention is to provide relative movementbetween a laser light beam and a surface to be measured so as to assurecomplete coverage of the surface in a reasonable time period.

An important objective of the invention is to provide a surfacemeasuring system which provides for automatic handling of the surfacesto be measured.

Since the invention leads itself to use in the semiconductor processingindustry, a preferred embodiment for use in that environment will bedescribed. In summary description, the invention consists of a surfaceinspection system comprising handling means for the manual transport ofsemiconductor wafers, automated means for loading and unloading saidhandling means without manual assistance, a laser light source, ascattered light detector, means for moving the laser, means for movingsemiconductor material while it is being exposed to the light from saidlaser, means for moving individual semiconductor material wafers to andfrom the position where they are subjected to exposure by said laserlight for purposes of detecting surface anomalies, and the necessarysignal processing and control circuitry to permit the automatedefficient operation of the system.

A significant difference between the present invention and that whichwent before is the ease of setup and maintenance in that the lightincident on the surface to be measured passes directly from laser sourceto that surface without interception by prisms, reflectors or otheroptical devices. The elimination of such intermediate optical devicesbetween laser and surface to be measured simplifies the establishmentand maintenance of the laser light beam perpendicular to the surfacesince the troublesome interaction among these optical devices does notpresent itself in the instant invention.

The laser movement means is unique in that it provides a moveablemounting platform for both the laser source and the scattered lightdetector and maintains the laser light beam perpendicular to the wafersurface while translating the point of impingement of the light beam onthe surface from a spot near the center of the semiconductor wafer tothe outer edge of the wafer.

Since mere lateral translation of the laser beam spot from the center tothe edge of the wafer would not provide for full surface coverage, asecond degree of motion is imparted to the wafer surface. When thesurface measurement is to be made, the wafer is held in position bymeans of a vacuum chuck. The vacuum chuck is designed to hold thesemiconductor wafer in position while it is controllably rotated, allthe while maintaining the wafer surface perpendicular to the laser lightbeam. When taken together, the rotary motion imparted to thesemiconductor surface by the vacuum chuck plus the lateral translationof the laser beam from center of the wafer to its outer edge provide forfull coverage of the semiconductor surface. The laser light beam scansthe surface in a pattern which forms an "Archimedes Spiral". Withsemiconductor wafers as large as five inches in diameter, this spiralscan, surface measurement is accomplished in less than four seconds.

While moving to and from the point at which the surface measurementtakes place, the individual wafers are transported on a cushion of inertgas or dry air to preclude contamination of the surfaces by handling inthe course of the measurement process.

The invention will best be understood after reading the detaileddescription which follows in which reference is made to the accompanyingillustrations in which:

FIG. 1 is an embodiment of the surface measuring system chosen forillustration in which the reflective surfaces are those of semiconductorwafers of the form normally encountered in semiconductor processing.

FIG. 2 is an exploded view of that portion of the surface measuringsystem which includes the collimated light source, its movable mount,the airway along which the semiconductor wafers are moved, and therotary work surface to which the wafer is affixed while the surfacemeasurement is made.

FIG. 3 illustrates the mechanism whereby the carriers in which thewafers are handled are oriented with the airway of the surface measuringsystem for purposes of unloading and loading said carriers.

FIG. 4 illustrates the motor drive mechanism whereby the collimatedlight source is laterally translated.

FIG. 5 illustrates the rotary chuck mechanism which is used for holdingand rotating the semiconductor wafers during the course of the qualityevaluation.

FIG. 6 shows a detail of that portion of FIG. 5 illustrating the mannerin which the vacuum is conveyed to the work surface of the rotary chuck.

FIG. 7 illustrates the airway for transporting the semiconductor wafersto the site at which the surface measurement is to be made. Alsoindicated are the means by which the wafer is located and oriented andplaced in position upon the rotary chuck surface.

FIGS. 8 and 9 are sectional views of the airway of FIG. 7 illustrated toimpart a clear understanding of the functioning of that device.

FIG. 10 depicts a side view of the airway of FIG. 7 permitting theillustration of mechanical details for stopping the wafer when itreaches the measurement site and for emplacing the wafer on the surfaceof the vacuum chuck.

FIGS. 11A and B provide a timing diagram illustrating the sequence ofoperations as a wafer progresses from input to output of the surfacemeasuring system.

FIG. 12 indicates the potential versatility of the measuring system asprovision is made to shunt measured wafers to a predetermined one of amultiplicity of wafer loading cassettes as determined by the relativemagnitude of surface imperfections found to exist on a givensemiconductor wafer.

The generalized concept of a surface inspection system for use inprocessing semiconductor wafers is depicted in FIG. 1. Fortransportation to and from the surface measuring equipment, the wafersare handled in cassettes 10. Cassettes 10 which have been loaded withwafers for processing are indicated by the reference numerals 101 and102 in FIG. 1. Cassettes 10 containing wafers which have passed throughthe surface inspection cycle are indicated by the reference numerals 103and 104. The operation of the equipment is such as to sort the wafers inaccordance with the magnitude of the measurement of their surfacequality. Thus, the surface quality of the wafers in loaded cassette 103will differ from those within wafer cassette 104, whereas, the waferswithin either of these two cassettes will fall within a designated rangeof measured surface quality. The cassettes are serviced by cassetteindexer 11. This involves unloading wafers from loaded cassettes 101 and102 for purposes of determing surface quality of the semiconductorwafers, and loading the surface inspected wafers into selected cassettes103 or 104 as determined by the magnitude of the surface qualitydetermined for each individual wafer. Cassettes 10 and cassette indexers11 are devices well-known in the semiconductor processing arts.

Cassette indexer 11 includes an elevation controlled platform 111 forpurposes of raising or lowering cassettes 10 so as to load or unloadwafers within said cassette. In the illustration of FIG. 1, a loadedcassette 101 is placed on elevation controlled platform 111 of cassetteindexer 11. At this time, elevation controlled platform 111 will belocated at its uppermost level. As the belt drive 112 of cassetteindexer 11 comes in contact with the lower surface of a wafer withinloaded cassette 101, the wafer exits from cassette 101, beingtransported by belt 112 to be deposited on airway 12 preparatory tohaving the wafer's surface inspected.

To the right of the illustration of FIG. 1 are shown partially loadedcassettes 103 and 104. When first placed on cassette indexers 11, thesecassettes were empty and devoid of any semiconductor wafers. When theempty cassette 10 was placed on elevator platform 111, said platform wasat its lowest level limit. In this manner, a wafer emerging from thesurface inspection process would be loaded in the topmost position inthe cassette.

Cassette indexers 11 are programmed to adjust the height of elevationcontrolled platform 111 so as to sequentially unload wafers from loadedcassette 101 of FIG. 1, thereby lowering cassette 101 each time a waferis withdrawn therefrom; and to sequentially load cassette 103 (or 104)as each wafer exits from the surface inspection process, thereby raisingcassette 103 each time a wafer is loaded. This function of loading andunloading semiconductor wafer cassettes using cassette indexers is notpeculiar to the instant invention and is well known to those familiarwith the art.

A novel arrangement of cassettes and cassette indexers is provided inthe surface measuring system illustrated in FIG. 1. Although the airway12 used to transport semiconductor wafers to and from the surfacemeasurement site is a single track device having provision to accept awafer from only one cassette indexer and the ability to output a waferto only one indexer, the illustration of FIG. 1 suggests that the systemmay be used with a multiplicity of cassette indexers located at both theinput and the output ends of airway 12. The invention provides a movablebase for indexers 11, such that the indexers may be translated laterallyso as to align belt 112 of a selected indexer 11 with airway 12. Inpractice, this means that at the input end of the system when loadedcassette 101, for example, is emptied of wafers, cassettes 101 and 102will be translated laterally so as to permit the wafers contained incassette 102 to be unloaded onto airway 12. While cassette 102 is beingunloaded, a new full cassette may be used to replace the now emptycassette 101. In this manner, a continuous supply of wafers is presentedat the input to airway 12 and no delay is encountered as the result ofthe emptying of a wafer cassette.

The ability to translate the cassette indexers has additional novelimplications. The surface inspection system provides a quantitativemeasurement of surface quality. That is, the output measurement value isrelated to some standard. As is typical in quality operations, a rangeof values centered about a desired standard will usually proveacceptable for the purpose for which the standard was established.Having measured the surface quality of a semiconductor wafer, it isdesired that it then be segregated from its companions in terms of itsability to fall within the range of measurement values dictated to bestandard or its failure to meet these acceptance criteria. Thus, in FIG.1, partially loaded cassette 103 may be taken as representative of thedestination of those wafers found to be acceptable in terms of theirsurface quality. Should a wafer fail to pass the surface inspection,partially loaded cassettes 103 and 104 would be laterally translated soas to cause the "defective" wafers to be loaded into partially loadedcassette 104.

Although FIG. 1 illustrates a simple "go, no-go" segregation of wafersat the conclusion of the surface inspection measurement, a moresophisticated means of segregating wafers at the output will bedisclosed which will permit the grouping of wafers according to amutliplicity of predetermined standards.

The means by which the cassette indexers are laterally translated havebeen designated as reference 13 in the drawings. Throughout thedrawings, like reference numbers are used for like components. Withrespect to FIGS. 1 and 3, the cassette translating mechanism 13 is seento comprise an air cylinder 131 which provides the necessary motiveforce to translate the indexers. A swiveled mounting flange 1311 is usedto affix air cylinder 131 to support base 14. Slide rails 132, alsomounted on base 14, are provided with slide blocks 1321 which arefastened to the undercarriage of indexers 11 so as to support saidindexers on said slide rails. The piston arm of air cylinder 131 isconnected by means of swivel flange 1312 to the undercarriage ofindexers 11. When air cylinder 131 is activated so as to cause itspiston arm to extend, indexers 11 will be translated so as to align, forexample, loaded cassette 102 with the input of airway 12. When aircylinder 131 is activated so as to cause its piston arm to retractwithin the cylinder, indexers 11 are translated so as to bring, forexample, cassette 101 in line with airway 12. A similar operativestatement can be made with respect to partially loaded cassettes 103 and104 of FIG. 1.

The heart of the surface measuring system is comprised of: 15, whichincludes the laser light source, scattered light detector, the means formounting same and maintaining the light beam perpendicular relative tothe surface of the wafer, and the translation drive which causes thelateral translation of the laser light source; 12, the airway assembly,which includes a wafer stop for positioning the wafer at the site atwhich the measurement will be made and a drop track section for loweringthe wafer into position upon the work surface of the vacuum chuckassembly 16.

A scattered light detector in the shape of an annulus is mounted to thenose of laser 152. Scattered light detector 151 is mounted such that thenose of laser light source 152 projects through the central opening ofthe annulus. Laser 152, with detector 151 in place, is mounted to laserplaten 153 by means of mounting plate 154. When so mounted, the lasernose and the associated light detector 151 project through a centralopening in laser platen 153. Laser platen 153 is supported by ball slideassemblies 155, which in turn are supported above airway 12 by ballslide blocks 156.

A centralized opening in airway 12 accepts the rotary spindle worksurface of rotary vacuum chuck assembly 16. Rotary chuck assembly 16 isshown in detail in FIG. 5 and FIG. 6. Chuck assembly 16 is comprised ofrotary vacuum spindle work surface 161, spindle shaft 162, bearings1631; which support spindle shaft 162 in spindle housing 163. A vacuumfitting 1632 is used to communicate a vacuum to work surface 161 viabore 1621. A horizontal bore 1622 in spindle 162, in communion withhorizontal bore 1633 in spindle housing 163 completes the vacuum pathbetween vacuum fitting 1632 and work surface 161. Vacuum seals 1634 areused to preserve the vacuum in the vacuum path existing between spindlehousing 163 and spindle shaft 162. Vacuum seals 1634 are maintained inposition by perforated spacer 1635.

DC servomotor 164 imparts rotary motion to work surface 161 by couplingmotor shaft 1641 to spindle shaft 162 by means of shaft coupler 1642. DCservomotor 164 is designed for rapid speed up, open loop operation.Optionally, it may be a hollow rotor motor. The lower end of motor shaft1641, as oriented in FIG. 5, bears a zero reference. Shaft encoder 165utilizes this shaft zero reference to provide encoder output signals, atoutput 1651, which may be used to determine the rotated disposition ofwork surface 161.

The surface of mounting flange 166 and rotary work surface 161 aremachined to run true and prallel to each other. When flange 166 isaffixed to the trued surfaces of mounting plate 17 (FIG. 1), the worksurface 161 of the rotary vacuum chuck assembly 16 will lie in a planeabove and parallel to the surface of mounting plate 17. When airway 12is mounted to mounting plate 17 by means of mounting blocks 127, rotaryvacuum work surface 161 is aligned with a central opening in airway 12so as to lie just below the travel path of wafers along the said airway.In this way, the wafers may travel down the path of airway 12 withoutmaking an interference contact with work surface 161. However, as willbe seen when airway 12 is discussed in detail, the clearance maintainedbetween the wafer on the airway and surface 161 is quite small. Mountingplate 17 also provides the support base for laser package 15. A pair ofprecision ball slides 155 is supported above the air track by means ofball slide blocks 156. Leveling adjustments are provided to assure thatthe movement of the ball slides is in a plane parallel to the surface ofmounting plate 17. Platen 153 bridges the gulf that exists between ballslides 155 and is supported on the upper surfaces of said ball slides.Leveling adjustments are again verified to assure that the surface oflaser platen 153 when translated by movement of ball slides 155, movesin a plane which remains parallel to the surface of mounting plate 17.This in turn assures that laser platen 153 will be laterally translatedin a plane which is parallel also to rotary work surface 161. Theleveling means indicated in the discussion are not illustrated in thefigures for the sake of clarity. Means for achieving this levelingfuntion will be well known to those versed in the art.

The movement which causes the lateral displacement of platen 153 iscommunicated by means of a lead screw drive which is coupled to DCsteppermotor 157 shown in FIGS. 1, 2, and especially FIG. 4. Lead screw1571 mates with a lead screw follower assembly on platen 153. Thisassembly is not shown. A spring damped coupler 1572 joins the shaft ofmotor 157 with lead screw 1571. A fly wheel 1573 is affixed to the leadscrew between the spring damped coupler 1572 and the lead screw followerassembly which is not shown. Since a steppermotor 157 is utilized toprovide the lateral translation of laser platen 153, it is possible totrack the movement of said platen. For example, assume that lead screw1571 and its lead screw follower assembly are such as to advance laserplaten 153 by 0.0003" per each step of the input drive signal. Assumethat the input signal drive is two hundred steps for each revolution ofDC steppermotor 157. Then for every two hundred steps at the input of DCsteppermotor 157, the motor will complete one revolution of lead screw1571 and cause the laser platen 153 to be laterally transported 0.006".By knowing the point at which drive is first applied to DC steppermotor157 and maintaining a count of the number of steps in the input drivesignal, the position of laser platen 153 with respect to that point atwhich drive was first applied will always be known.

A second order damped system is produced by the use of spring dampedcoupler 1572 and fly wheel 1573. The coupler smooths the effect of theincremental motion of the shaft of steppermotor 157 while fly wheel 1573reduces the torsional resonance of that motor.

Laser 152 with scattered light detector 151 mounted on its nose isaffixed to laser platen 153 by means of laser mounting plate 154. Anopening in laser platen 153 permits the passage of a beam of laser lightfrom laser 152. Adjustments, not shown, are provided to establish thelaser beam on a path perpendicular to the surface of laser platen 153.Adjustments for this function are well known in the art. Establishingthe laser light beam on a path perpendicular to laser platen 153effectively causes the light beam to strike work surface 161 at anincidence angle of ninety degrees. With the beam established at ninetydegrees to work surface 161 plus the ability to translate that beamwhile work surface 161 is rotated, the stage is set for the scatteredlight inspection of the surface of a semiconductor wafer.

Reference should now be made to the sheet containing FIGS. 7 through 10inclusive. Airway 12 and parts thereof are illustrated in this group ofFigures. FIG. 9 is a cross-sectional view taken as indicated in FIG. 7.Although the word "air" will be used in description of airway 12, itshould be borne in mind that any appropriate gaseous medium such as dryair or an inert gas may effectively be used to transport thesemiconductor wafer.

In FIG. 7, it is seen that air vents 121 located in a top surface ofairway 12 form two parallel rows which effectively define an airtrackdown which a semiconductor wafer may travel. In the cross-sectional viewof FIG. 9, it is seen that airway 12 is constructed so as to define anair plenum 1211. A supply of air to said plenum is introduced at airinput 1212. Air introduced at input 1212 fills plenum 1211 and exitstherefrom by flowing out vents 121. This flow of air out vents 121, issufficient to support a semiconductor wafer between side walls 1213which further define the airtrack down which said semiconductor waferstravel on airway 12. Airway 12 is comprised of three sections asillustrated in FIGS. 7 and 10: a left-hand fixed section 122; a central,movable, drop track section 123; and a right-hand fixed section 124. Intransiting airway 12, a wafer will enter at the left side of airwaysection 122, travel across central section 123, and exit at theright-hand edge of section 124. As may be seen in cross-sectional viewFIG. 8, the flow of air from plenum 1211 out air vents 121 is directedsuch as to cause the air to flow from left to right along the airtrackof airway 12 in the illustration of FIG. 7. It is this left to rightflow which supports the wafer and transports it along the airway definedby the multiplicity of vents 121 and side edges 1213.

A semiconductor wafer entering airway section 122 will be transported onthe airtrack until it is detected at sensor 1221. When the presence ofthe wafer is detected by sensor 1221, vacuum stop 1222 is activated. Theeffect of activating stop 1222 is to halt the movement of a wafer acrossthe airtrack of airway 12. While the wafer is held fixed by said vacuumstop sensor 1221 alerts the system controls that a wafer is awaitingprocessing. The wafer will be held at vacuum stop 1222 until the systemascertains that there is a clear track ahead. With a clear track ahead,vacuum stop 1222 is then inactivated and the wafer proceeds on itsjourney to the right until it is brought to a halt by wafer stop 1231.

With its travel halted by wafer stop 1231, the wafer is positioned abovea central opening 1232 in airway 12. While so positioned, it issupported on the flow of air exiting from vents 121. In the assembledsurface inspection system, rotary work surface 161 is positioned incentral opening 1232 of airway 12 such that a wafer may progress alongthe airtrack just avoiding interfering contact with the work surface161. Thus, with wafer stop 1231 positioned so as to impede the progressof the wafer down the airtrack of airway 12, the wafer will be held inposition directly above the work surface 161.

Once a wafer has been released by vacuum stop 1222, sufficient time ispermitted to allow the wafer to travel along section 123 so as to reachwafer stop 1231 and to settle down into stable contact with said waferstop. When sufficient time has elapsed to cause the settling of a waferagainst wafer stop 1231, a vacuum is communicated to rotary work surface161. Section 123 of airway 12 is then caused to move in such a manner asto lower the wafer into intimate contact with work surface 161 where thewafer becomes readily affixed by reason of the vacuum communicated tothat surface. Wafer stop 1231 is then moved so that it no longer makesan interference contact with the edge of the wafer.

To understand how the motion is imparted to both section 123 of airway12 and to wafer stop 1231, reference should be made to FIGS. 7 and 10.Here it may first be noted that drop track section 123 is supported bypivot rod 1233. It is maintained in this position by the action of droptrack cylinder 1235 which is shown in FIG. 10 as having its fixed endsupported to the undercarriage of fixed section 124 of airway 12 whileits piston arm is affixed by a clevis arrangement to the undercarriageof drop track section 123. Retraction of the piston arm of air cylinder1235 will cause drop track section 123 to pivot about pivot rod 1233 ina counter-clockwise direction such that the left-hand end of section 123is lowered. A set screw 1223 and stop bracket 1237 arrangement permitsthe drop track to have its surfaces adjusted to be flush with theassociated surfaces of fixed section 122 when air cylinder 1235 isactivated so as to extend its piston arm and thereby raise the left-handend of drop section 123.

A similar pivot rod 1234 is provided, in cooperation with wafer stop arm1238, to raise and lower wafer stop 1231. Motion is imparted to wafer1231 by operation of air cylinder 1236. The fixed end of air cylinder1236 is fastened to the undercarriage of drop track section 123. Thepiston arm of air cylinder 1236 is fastened to wafer stop 1238 by meansof a clevis arrangement. Operating wafer stop cylinder 1236 so as tocause the retraction of the piston arm causes the lowering of wafer stop1231 as it is forced to pivot counter-clockwise about pivot rod 1234.Although in its raised position wafer stop 1231 will make contact with awafer traveling along the airtrack of airway 12; lowering wafer stop1231 will remove that contact.

With the drop track lowered, the vacuum on vacuum chuck assembly 16, awafer in intimate contact with the work surface 161 and affixed there bythe vacuum, and finally with the wafer stop arm 1231 retracted, thewafer is free to be rotated with rotary work surface 161.

Semiconductor wafers, while generally circular, typically have one ormore segments removed so as to produce what is known as a flat alongsome portion of its curved outer edge. It is possible to use this edgeto obtain a reference position on the surface of the wafer as it isrotated by rotary work surface 161. To this end, wafer-flat sensor 1224is suggested in FIG. 7 for the purpose of detecting the discontinuityintroduced by the flat in the smooth, continuous curve at the peripheryof the wafer.

At the point in time we have now reached in our discussion of thetransit of airway 12 by a semiconductor wafer, DC servomotor 164 will beenergized. This will cause the rapid rotary acceleration of work surface161. At the same time, drive signals will be applied to steppermotor 157so as to cause the laser beam, which is now functioning, to transitacross the surface of the wafer undergoing surface inspection. Byproperly establishing the initial position of the laser, the laser beamwill transit across the face of the wafer reaching the center of thewafer at the time that servomotor 164, and therefore work surface 161,has reached its maximum rotary acceleration. From that time, the outputof scattered light detector 151 is determined and correlated with therotated position of the semiconductor wafer under test and thetranslated position of the laser beam. By correlating these readingswith respect to the location of the flat at the edge of thesemiconductor wafer as determined by wafer-flat sensor 1224, the actuallocation of surface anomalies may be pinpointed on the wafer.

With the surface inspection of that particular wafer completed, thedrive to the rotary work surface 161 is discontinued, the vacuum supplyto that work surface is terminated and drop track section 123 is movedto its raised position thereby floating the wafer from rotary surface161 on the flow of air exiting from air vents 121. Wafer stop 1231 ismaintained in its depressed position and the wafer is, therefore, freeto travel unimpeded to the right along the airtrack of airway 12. As thewafer traverses the fixed section 124 of airway 12, its exit therefromis sensed by clear-track sensor 1241. With the track clear, wafer stop1231 is again moved to its raised position and a new wafer, which may beawaiting tests at vacuum stop 1222, is released to travel down untilimpeded by wafer stop 1231. The test cycle then repeats itself.

The test cycle may be readily understood by referring to FIGS. 11A and11B. In FIG. 11A, the various steps of each cycle and their timing arespelled out. In FIG. 11B, three wafers are shown being cycled along theairtrack of airway 12. The movement of each of these wafers; W0, W1, andW2; along the airtrack of airway 12 may be correlated by their relativeposition with respect to the timing diagram of FIG. 11A.

The arrangement of cassettes 103 and 104 at the output of airway 12provides a simple go, no-go segreation of tested wafer surfaces. FIG. 12suggests that the inclusion of an additional airway section, eg. 125between output cassettes 103 and 104, permits the addition of otheroutput cassettes: 105, 106, 107 . . . 10n; each pair of cassettes havingan airway section 126, 127 . . . 12n, associated. In this manner, theinspected wafers may be segregated with a finer granularity with respectto the measured surface quality.

When a wafer is measured and its surface quality determined, the outputcassette, eg 105, associated with that determined surface quality willbe laterally translated so as to be in-line to receive the wafer as itexits from the measuring process.

Although the output cassettes are shown arranged in pairs no limitationis implied thereby and other arrangements may be employed.

What we have disclosed is a surface measuring system which uses lightscattered from surface anomalies to profide a quantitative measurementof surface quality. Because the collimated light travels from lightsource to the surface to be measured without impinging upon reflectivesurfaces, prisms, or other optical devices to bend or deflect its path,the system is relatively easy to set up and maintain. The means by whichthe collimated light beam is moved across the surface undergoingmeasurement is such as to provide full coverage of the surface and topermit correlation of the beam location on that surface with the sensedindication of surface anomalies. The physical system has been describedwithout indication of signal processing or control circuitry which thosefamiliar with the art will be well familiar with, knowing that variousposition sensors indicated in the foregoing disclosure are required aswell as sensors for indexing the initial and final positions of thelaser light beam. The rotary drive motor is a stepperdrive motor and themethod of determining the rotated position of the disc has been setforth in the foregoing discussion. Means for sampling the output of thescattered light detector with reference to the determined position ofthe light beam are well known. While such processing and controlcircuitry may in themselves be innovative to the point of patentable, itis not necessary that such unique circuitry be employed in practicingthe invention disclosed herein.

Having described our invention in such clear terms as to enable thoseskilled in the art to understand and to practice it, that which we claimas worthy of the grant of Letters Patent is:
 1. A scattered lightsurface measuring system for the detection of surface anomalies onreflective surfaces of flat articles to be tested, comprising:a supporthaving a horizontal upper surface to support a flat test article withits upper reflective surface in a horizontal position; a collimatedlight light source mounted above the support and positioned and arrangedto transmit a collimated light beam directly perpendicularly upon thehorizontal reflective surface in the absence of interception byreflectors, prisms, lenses, or other optical path bending devices; ascattered light detector surrounding the collimated light beam above thesupport to intercept the light rays scattered by the surface anomaliesof the test article; the support for the test article is controllablypowered to rotate continuously about a vertical axis as required; thelight source is mounted to travel transversely over the support and testarticle while maintaining the perpendicularity of the collimated lightbeam to the surface of the test articles; controllable power means isprovided to produce the transverse travel of the light source; and thecontrol means are operable to cause simultaneous rotation of the testsample and transverse travel of the light source between the center andthe perimeter of the test article and produce impingement of the lightbeam upon the entire area of the test sample in the form of anArchimedean spiral; and the support for the test article has aperforated upper surface and a controllable vacuum pump is connectedthereto through the interior of the support to constitute a vacuum chuckfor holding the test article securely in place while the chuck isstationary or rotating.
 2. A scattered light surface measuring systemfor the detector of surface anomalies on reflective surfaces of flatarticles to be tested, comprising:a support having a horizontal uppersurface to support a flat test article with its upper reflective surfacein a horizontal position; a collimated light light source mounted abovethe support and positioned and arranged to transmit a collimated lightbeam directly perpendicularly upon the horizontal reflective surface inthe absence of interception by reflectors, prisms, lenses, or otheroptical path bending devices; and a scattered light detector surroundingthe collimated light beam above the support to intercept the light raysscattered by the surface anomalies of the test article; the support forthe test article is controllably powered to rotate continuously about avertical axis as required; the light source is mounted to traveltransversely over the support and test article while maintaining theperpendicularity of the collimate light beam to the surface of the testarticles; controllable power means is provided to produce the transversetravel of the light source; the control means are operable to causesimultaneous rotation of the test sample and transverse travel of thelight source between the center and the perimeter of the test articleand produce impingement of the light beam upon the entire area of thetest sample in the form of an Archimedean spiral; loading means areprovided for moving successive test articles from a storage location tothe vicinity of the rotatable support for test; unloading means areprovided for moving the articles from the test location to a secondstorage location; the loading and unloading means include an air trackprovided with jet means for conveying the articles along the track; anintermediate section of the track is formed with an aperture locatedabove the rotary support at the test location; stop means is mounted onthe track to intercept each article above the aperture; the intermediatesection of the track is vertically movable with respect to the othersections; and means are provided to lower the track and deposit a testarticle on the rotary support for the test operation free ofinterference by other parts of the apparatus.
 3. The measuring system ofclaim 2 said loading means further comprising:input cassette meanshaving at least an input cassette for housing an uninspected pluralityof said test articles; and input indexing means for selectively shiftinga selected one of said uninspected plurality of said test articles ontosaid track.
 4. The measuring system of claim 2 said unloading meanscomprising:lifting means for raising said portion of said track so thatsaid test articles is thereby lifted from said rotary work surface; andrelease means for removing the impediment of said stop means so thatsaid jet means can further propel said test article along said track. 5.A scattered light surface measuring system for the detection of surfaceanomalies on reflective surfaces of flat articles to be tested,comprising:a support having a horizontal upper surface to support a flattest article with its upper reflective surface in a horizontal position;a collimated light light source mounted above the support and positionedand arranged to transmit a collimated light beam directlyperpendicularly upon the horizontal reflective surface in the absence ofinterception by reflectors, prisms, lenses, or other optical pathbending devices; and a scattered light detector surrounding thecollimated light beam above the support to intercept the light raysscattered by the surface anomalies of the test article; said scatteredlight detector has an annular shape and further has a disposition aboutsaid beam of collimated light so that said beam of collimated lightpasses undetected through the center of the annulus while reflectedlight scattered by a surface anomaly is detected.